2012 mid-August came a tip-off from a visual observer that the USA 217 satellite and the Minotaur rocket that had launched it were tagged wrongly in the unofficial catalogue for classified objects.

The catalogue contains items that have been identified by amateur observers - radio and visual - and for which SpaceTrack does not publish orbital data. The contact also said that there was an unconfirmed report of USA 217 transmitting at 400.180 MHz (see Footnote). It is not a frequency that had been checked from the Zarya location in the past.

Looking for Signals

Overnight 2012 August 15-16 UTC offered several passes by the item identified as the Minotaur (2010-062G/37228) but none of them yielded signals. The same was true of an afternoon period during August 16. Finally, about 24 hours after first looking, a transmission with the familiar Doppler curve shape turned up.

Observation

The signal faded-in 2012 August 16 at 23:35:46 UTC and ceased suddenly at 23:43:15 while the satellite was still well above the horizon.

A measure of the Closest Approach time from the Doppler curve gave 23:41:06 UTC. A simulation using the latest available, but 10 days old, element set for the Minotaur predicted it four seconds earlier. It was pretty well certain to be that object, confirming it was indeed USA 217 under an erroneous ID.

The signal's content consisted of short bursts of data transmitted at regular intervals. It is reminiscent of the 'trigger' transmissions from Russia's Gonets and Strela-3 store-dump communications satellites. It indicates that the satellite serves a similar purpose but it seems the trigger signal is switched on only for short intervals rather than providing worldwide access.

One other thing gleaned from the observation was that the interval between trigger pulses is 16.0 seconds ±0.1s.

The transmission almost certainly comes from the Ocean Data Telemetry Microsat Link payload, described as being designed "to extract data from widely distributed sensors such as ocean data buoys or unattended ground sensors". It was developed by a US company called Praxis Inc under a contract with the Office of Naval Research. Documentation at the Praxis web site indicates that similar payloads were installed on TacSat 3 (2009-028A/35001) and TacSat 4 (2011-052A/37818).

In this particular instance, USA 217 had approached the UK from the direction of Greenland via Iceland. Switch off was over the North Sea so it could have been attempting to pick up data from a remote sensor in Arctic waters.

Two further, isolated, transmissions occurred on August 17 from that same area, but on differing ground tracks. it suggests the tract of ocean between Scotland and Iceland is where one or more sensors currently holding USA 217's attention are drifting, or anchored. There is no indication of the sensors' purpose.

Footnote

Probably not a coincidence - almost simultaneously with the 'tip-off' - a discussion thread started on the Seesat-L observer forum about updating the identities of USA 217 and its rocket because a long-period detailed analysis of visual observations had indicated that they might have been tagged incorrectly. The analysis did not include any reference to radio transmissions reported by radio observer Maik Hemenau as coming from the object carrying the 'Minotaur' identity.

A single, simple, radio observation has proved the point almost in an instant and the new identity can now be marked as "Certain" rather than Seesat-L's "probable".